Yeah, with a flat (center-cut) they're trimmed lean. Without the fat to baste the meat and less mass to cook, it cooks faster and with more meat surface exposed. It's a trade-off of sorts in that with lean trimmed brisket you have more meat exposed to smoke, more exposed meat surface to form that bark on, but no fat to render out and baste the meat. There are tricks to compensate for the extra bare meat to retaining more natural moisture, but increased cook chamber humidity is not one of them. Same goes for creating a nice bark and preserving it while it rests.
I don't smoke brisket very often these days, but I always enjoyed a nice big ol' packer. They're easy to smoke and versatile in what you can transform them into, such as burnt ends or pulled beef from the point, and sliced from the flat. Burnt ends are a treat, and any time I smoke brisket I get frowns if I don't smoke up some burnt ends from the point. Burnt ends are the first thing to disappear.
If you do burnt ends, I recommend that you pull the brisket out and separate the point/flat muscles when the point is still in the mid-150* temp range. Once separated, rest briefly, then cube it up into about 1-1/4 - 1-1/2" chunks, dust with more rub and toss them in a bit of sauce (one of the few times I ever use sauce on my smoked meats) and return to open grates for a second round in the smoke. Cook for another 2-3 hours low & slow...a quick peek in the smoker for color after 2 hours will tell you. When they get a deep brown, almost scorched color, they're ready. They get a nice, crisp bark with a light popping but tender chew, and very good natural moisture...no dipping sauce needed, they're that good. But that's my target texture ever since I learned that the lower-temp separation gives so much better finished burnt ends.
I wasn't going to post this earlier, but maybe this will help you understand developing and preserving bark, as well as natural moisture retention a little better...this is used with lean-trimmed meats, but is also beneficial for meats with fat-cap intact or lightly trimmed. Look for the brisket...for a killer bark, look for the pork butt or picnic...that best explains how to preserve the bark:
Wet-to-Dry Smoke Chamber Method
Eric